This invention relates to improvements in surface treating agents for improving the cleaning of the surfaces of semiconductors and LCD.
Particularly, the present invention relates to improved surface treating agents and an improved treating process which are used for cleaning to remove, for example, contamination with organic materials, fine particulates and metals on the surface of silicon, germanium, and compound semiconductors (e.g. Ga--As, Ga--P, etc.), which are often used for the production of semiconductors, integrated circuits, LCD, etc., the surface of films contacting with semiconductors, or at least one surface of glass substrates, and are thus effective in improving the surface cleaning to increase the yield of products.
At present, semiconductor devices such as LSI, IC, diodes, commutators, etc. are mostly silicone devices which are produced by subjecting a silicon wafer to processing steps such as vapor phase growth, oxide film formation, impurity diffusion, vapor phase deposition of electrode metal, etc.
Since contamination with impurities has remarkable undesirable influences on characteristics of semiconductors, it is necessary to clean silicon wafer surfaces sufficiently for removing contamination prior to the above-mentioned processing steps. Various industrial cleaning methods which are different as for treating chemicals, treating temperature, etc. have been employed. A series of treatments called "RCA cleaning method" (RCA Review pp. 187-206, June, 1970, etc.) appeared in the mid-1970s and have been used widely since that time and become a major method for wet cleaning. The RCA cleaning method is a combination of (i) SC-1 treatment using ammonia, hydrogen peroxide and water and effective in removing contamination with organic materials and a part of metals such as Cu, Ag, etc., particularly effective in removing contamination with fine particulates, (ii) a dilute HF treatment for removing natural silicon oxide films, and (iii) SC-2 treatment using hydrochloric acid, hydrogen peroxide and water, and remarkably effective for removing metal contamination.
The sequence of RCA cleaning method generally used is SC-1 treatment.fwdarw.dilute HF treatment.fwdarw.SC-2 treatment. But the dilute HF treatment easily gives fine particulates contamination on the surface of silicon wafer and the SC-2 treatment is poor for removing fine particulates. With recent ultra-high integration of LSI, device patterns are rapidly miniaturized and the size of fine particulates which damage devices is also remarkably miniaturized. The smaller the size of fine particulates becomes, the stronger their adhesive strength for wafers becomes. Thus, the removal of fine particulates becomes more difficult, so that the production yield is strongly dependent on the ability to remove fine particulates contamination at the time of cleaning. In order to remove the particulates more effectively, attempts were made to conduct the SC-1 treatment at the last step of cleaning sequence. But it is known that metallic impurities are adsorbed on the surface of a wafer subjected to such a cleaning sequence and cause troubles such as deterioration in oxide breakdown voltage and abnormal growth of an oxide film formed by thermal oxidation.
Metallic elements detected on the surface of wafer after the SC-1 treatment are Fe, Al, Ca, Mg, Zn, etc. At first these contaminating metals are derived from the production apparatus for the processing step before the cleaning step or derived from circumstances. They consist of the elements which are not removed due to insufficient cleaning ability of SC-1, and the elements adsorbed on the wafer from the treating solution including these metal elements.
When the SC-1 treatment is employed, Fe and Al are particularly liable to be adsorbed and retained and are difficult to remove, as compared with the other elements. In a mass production factory, as a high-productivity cleaning apparatus, there is commonly used a Tact method wherein wafers entered in a carrier are passed to cleaning baths one after another. Thus, contaminating elements released in the liquid of a SC-1 treating bath by cleaning are gradually accumulated to provide an undesirable contamination by adsorption on succeeding wafers. Further, since Al is used as a wiring material in semiconductor devices, contamination with Al is unavoidable. In addition, when ammonia and hydrogen peroxide with extremely high purity are not used, there takes place contamination by adsorption of impurities in the cleaning chemicals. In the production and storing of hydrogen peroxide, since metals such as Al are used, it is particularly necessary to highly purify the hydrogen peroxide used for semiconductor cleaning. Further, these highly pure chemicals are easily contaminated in vessels for transport or in feeding system to cleaning baths, so that it is not easy to maintain the purity of these chemicals at very high level in cleaning baths.
Usually, after the SC-1 treatment, the surface of wafer is contaminated with about 10.sup.11 to 10.sup.12 atoms/cm.sup.2 of Fe, about 10.sup.11 to 10.sup.13 atoms/cm.sup.2 of Al, and about 10.sup.10 to 10.sup.11 atoms/cm.sup.2 in the cases of Ca, Mg and Zn. Of such metal contaminations, the Fe contamination shortens the lifetime at a Fe concentration of 10.sup.11 atoms/cm.sup.2 or more, and the Al contamination causes abnormal growth of an oxide film at an Al concentration of 10.sup.12 atoms/cm.sup.2 or more. Therefore, it is necessary to make the concentrations of Fe and Al as low as about 10.sup.10 atoms/cm.sup.2 and about 10.sup.11 atoms/cm.sup.2, respectively. It was extremely difficult to obtain such a degree of cleanness only by the SC-1 treatment.
As another cleaning method similar to the SC-1 treatment, there is known a method using an organic alkali and hydrogen peroxide. For example, there are disclosed tetramethylammonium hydroxide (TMAH) and hydrogen peroxide (Japanese Patent Unexamined Publication No. 50-147284), trialkyl(hydroxyalkyl)ammonium hydroxide and hydrogen peroxide (Japanese Patent Examined Publication No. 53-43012, U.S. Pat. No. 4,239,661, U.S. Pat. No. 4,339,340), etc. These methods are excellent in fine particulates removing ability which is characteristic of treatment with alkali and hydrogen peroxide, like the SC-1 treatment. But the methods cause serious adsorption of Fe, Al, etc. from the treating solution and hence are not sufficient in cleaning ability for wafers contaminated with Fe, Al, etc. A method of catching metallic impurities as stable water-soluble complex to make them inactive to a material to be cleaned is a conventional method for formulating usual cleaning agents. For example, addition of a complexing agent to TMAH and hydrogen peroxide is disclosed in Japanese Patent Unexamined Publication No. 50-158281, and addition of a complexing agent to trialkyl(hydroxylamine)ammonium hydroxide and hydrogen peroxide is disclosed in Japanese Patent Examined Publication No. 53-20377, U.S. Pat. No. 4,239,661 and U.S. Pat. No. 4,339,340. In each case, the added amount of the complexing agent should be 0.01% by weight or more. Addition of a cyano compound is dangerous. When an organic compound such as ethylenediamine tetraacetic acid (EDTA), triethanolamine, etc. is added in an amount of as much as 100 ppm as described in them, harmful carbon contamination takes place on silicon surface to cause problems in electrical properties.
The "alkali and hydrogen peroxide" cleaning very effective against contamination with fine particulates is not sufficient by itself in cleaning efficiency for harmful metallic impurities such as Fe and Al. The method of adding a large amount of a complexing agent deteriorates electrical properties and hence is not practical. As a method for solving this problem, addition of a phosphonic acid type complexing agent to an alkali and hydrogen peroxide in an amount of 10.sup.-3 % by weight or less has been reported in Japanese Patent Unexamined Publication No. 5-275405 (U.S. Pat. No. 5,290,361). In the case of using a phosphonic acid type complexing agent, when its added amount is 10.sup.-3 % by weight or less, a wafer treated by such a method does not show an undesirable influence of contamination with organic materials on electrical properties, and the Fe concentration can be limited to about 10.sup.10 atoms/cm.sup.2. However, the complex formation rate for Al becomes slow, resulting in insufficient effect. Therefore, the Al concentration is at least about 10.sup.11 atoms/cm.sup.2 on the treated wafer surface. It is known that contamination of silcon surface with Al affects the growth rate of an oxide film during thermal oxidation. This is not desirable from the viewpoint of precise control of the process.
As described above, the "alkali and hydrogen peroxide" cleaning very effective against contamination with fine particulates is not sufficient in cleaning efficiency for Al, one of the harmful metallic impurities. An effective means for solving this problem has not yet been found.